Fermentation butyric acid, butanol, acetone

More than seventy years ago the impressive discovery was made that bioreduction of mannitol, glycerol and starch yields butanol. Fermentations in which butyric acid, butanol and acetone are formed from carbohydrates by different bacilli (butyl bacteria) belong in this group. The term butyl bacteria as a generic name for microbes producing the genetically related substances of the four-carbon series was proposed in 1921" and has been applied since then. The approximate course of these reactions is shown by the following formulations which, however, do not explain the mechanism ... 

Reductive fermentations can be classified into the following types (1) butyric acid fermentation, (2) butanol-acetone fermentation, (3) butanol-isopropyl alcohol fermentation, (4) butylene glycol-ethanol fermentation, and (5) acetone-ethanol fermentation. 

The butanol-acetone fermentation, using whey supplemented with yeast extract, is substantially a butanol fermentation. Volatile solvents from the fermentation consist of 80% butanol, 13% acetone, and 5% ethanol. Approximately 30% of the lactose that is fermented is converted to butanol, 5% to acetone, 2% to ethanol, and the balance largely to CO2 and small quantities of butyric and acetic acids, acetyl-methylcarbinol, and hydrogen. 

Polyhydroxybutyric acid is a storage compound for excess carbon in many microorganisms (E 2.2). It may be used in the production of plastics (F 4). Acetoacetic acid, acetone, and /S-hydroxybutyric acid are excreted in the urine of people with a pathologically high blood sugar level (diabetes mellitus) (E 1). Their appearance is of diagnostic value. Butyric acid, butanol, and acetone are products of microbial fermentations. 

Microorganisms have also been developed to produce alternative products, such as lactic acid [65], propane-1,3-diol [67], 3-hydroxypropionic acid [68], butane-2,3-diol [69] and numerous other intermediates. For instance, bacteria such as the Clostridium acetobutylicum ferment free sugars to C4 oxygenates such as butyric acid or butanol. They form the C4 oxygenates by Aldol condensation of the acetaldehyde intermediates. The Weizmann process exploits this property to ferment starch feedstock anaerobically at 37 °C to produce a mixture of w-butanol, acetone and ethanol in a volume ratio of 70 25 5 [3],... 

After 606 h, the dilution rate in reactor L was decreased from 1.2 to 0.6 hr1. When the pH was adjusted to 3.5 at 623 h, the concentrations of acetone, butanol, and ethanol decreased dramatically and reduced to almost 0 g/L by 700 h (see Fig. 4A). In the meantime, the concentrations of glucose and butyric acid increased with time to reach almost their feed concentrations. Coupled with the low OD value, these findings make it apparent that the fermentation had almost ceased at this low pH value. 

Species of Clostridium have been used on a very large scale for the production of industrial solvents such as acetone and butanol. Many clostridia ferment sugars with the formation of carbon dioxide, hydrogen and butyric acid. Some of them convert butyric acid to butanol and the acetic acid to ethanol and acetone. The acetone-butanol process by Clostridium acetobutylicum expanded after its introduction just before World War I. Although the industry has now been almost totally replaced by the synthetic production from oil, it may again be of interest for the production of renewable monomers. 

Product Challenged Growth Studies. To study the inhibitory factors of the acetone-butanol fermentation, the growth rates of Cl. acetobutylicum in the presence of each fermentation product were determined. The end products used in this study included ethanol, butanol, acetone, acetic acid, and butyric acid. From the slopes of the least squares regression lines of optical density vs. time data, the maximum specific growth rates in the presence of varying concentrations of each inhibitor ()j ) were determined. The results for each fermentation product are shown in Figures 1 - 3. There appears to be a threshold concentra-... 

P data. The inhibition consteints for etheuiol and acetone are approximately ten times greater than that for butanol, acetic acid, and butyric acid. This is indicative of the relatively low toxicity of acetone and ethanol as compared with the other fermentation products. 

As described above, the clostridial ABE fermentative path leads to synthesis of butanol, together with smaller amounts of acetone, ethanol and acetic and butyric acids, together with carhon dioxide and hydrogen (Branduardi et al. 2014). Normally, the solvent ratio of acetone, butanol and ethanol, respectively, is 3 6 1, and the total solvent concentration is around 20 g/L (Connor and Liao 2009). Many natural clostridial strains have the upper butanol tolerance limit at about 11-12 g/L. However, some mutants and engineered strains can tolerate up to 19 g/L of butanol (Jang et al. 2012a). 

Acetyl-CoA is regenerated in this process. The overall product yields in moles per mole of glucose converted are approximately 0.5 acetate, 0.75 butyrate, 2 CO2, and 2 H2 2.5 mol ATP are formed. The nonacidic compounds, acetone, 1-butanol, and 2-propanol, are formed by transformation of some of the acetoacetyl-CoA into acetoacetic acid, which is the precursor of acetone and 2-propanol. Some of the butyryl-CoA is the precursor of 1-butanol via intermediate butyraldehyde. Ethanol is formed by reduction of small amounts of acetyl-CoA. The end result of the production of the neutral products by these additional pathways is that the yields of the other products are reduced. The neutral products are in a lower oxidation state than the acidic products and require additional reducing power as NADH to be formed. Some of the product Hj serves to sustain and provide NADH because higher partial pressures of H2 during the fermentation promote higher yields of the neutral products, whereas removal of the product H2 as it is formed has the opposite effect. 

Over the years the butanol or the other minor solvents became more important than the acetone, and the process continued to develop. Different species of clostridia were used to produce these other solvents (e.g. C. butylicum for butanol and isopropanol, and C. butyricum for butyric and acetic acids). Strains were isolated which made a very efficient use of molasses rather than starch. The pH of the medium was controlled by aqueous ammonia rather than by chalk, giving a completely soluble medium suitable for in-line sterilisation. The spent cells recovered at the end of the fermentation were found to be a good source of riboflavin (Vitamin B2), and, after drying, were used as a supplement to animal feeds. The carbon dioxide and hydrogen were collected and sold, and continuous distillation was developed for the solvent recovery. 

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